timer: cortex_m_systick: use direct interrupt handler instead of C-function

The current driver implements the global defined "systick" interrupt
callback "sys_clock_isr" as a standard C function with an argument.
However, ARM's direct interrupt handlers do not have any arguments;
they must be declared as "void handler(void)".

Additionally, the direct handler should include some missing special
header/footer.

Fixes: #75693

Signed-off-by: Andrey VOLKOV <andrey.volkov@munic.io>